1. Field of the Invention
The present invention relates to a circuit for zero voltage-zero current switching in a full-bridge DC-DC converter, and more particularly to a circuit for zero voltage-zero current switching that can achieve a significant reduction in ripples of an output current as well as zero voltage-zero current switching with only passive minority-carrier elements on the secondary side in a full-bridge DC-DC converter.
2. Brief Description of the Prior Art
Because both voltage and current switching transitions have time lapse during the switching operation of power semiconductor elements, when a switch turns ON or OFF, there is a time interval where both current and voltage are applied to the switch simultaneously, resulting in a power loss during the time interval. In particular, a switching device such as an IGBT (Insulated Gate Bipolar Transistor) or a GTO(Gate Turn Off thyristor) has a large turn-off switching loss because a tail current flows through the switching device during time interval (shown by `L` of FIG. 7) after voltage has been applied across the switch, as shown in FIG. 7.
The maximum switching frequency is usually limited because such a switching loss increases in proportion to the frequency. Accordingly, in order to reduce the switching loss and to achieve high-frequency switching operation of such a switching device with the above-mentioned characteristics, either a zero current switching scheme (FIG. 8A) or a zero voltage switching scheme (FIG. 8B) is usually used. After freewheeling current flows through a diode connected in reverse-parallel with a switching device and then the voltage applied across it becomes zero, when the switching device is turned ON, the switching operation at zero voltage is performed, so there is no switching loss, as shown in FIG. 8A. On the other hand, a switching loss still occurs when the switching device turns OFF; this is shown in FIG. 7. Connecting a snubber capacitor across the switching device can slow down the rising slope of the voltage across the switching device, as shown in FIG. 8A, thereby reducing the turn-off switching loss.
If a switching device turns OFF when current flowing through it is zero, there is no energy loss caused by turn-off because stored minority carriers generating the tail current have disappeared. When the switching device turns ON, a switching loss still occurs due to the time duration required for reverse-recovery of the diode. However, because there is no reverse-recovery of diodes in a full-bridge DC-DC converter, there is a negligible switching loss caused by turn-on; consequently, the zero current switching is preferable to zero voltage switching in the full-bridge DC-DC converter.
Currently, a full-bridge DC-DC converter with zero voltage switching is used for high-power and high-frequency switching. In addition, a snubber capacitor with a large capacitance cannot be connected in parallel with the switching device because zero voltage switching can be used in a situation where a load range is narrow. Therefore, switching devices using a minority carrier such as IGBT or GTO have many problems resulting from a large switching loss during the high-frequency switching operation.
Full-bridge DC-DC converters capable of performing both zero-voltage switching and zero-current switching, as shown in FIG. 6, have been proposed to enable a high-frequency switching operation with those switching devices using minority carriers.
The full-bridge DC-DC converter of FIG. 6 comprised of the left leg of switch S1 and S3 on the primary side for performing zero-voltage switching and right leg of switch S2 and S4 for performing zero-current switching under the large range of the load, is capable of performing a high-frequency switching operation with switching devices, such as IGBT and GTO, where the tail current flows through when turned off, and reducing the energy loss of the transformer T and switches S1, S2, S3, and S4 on the primary side by having the freewheeling current of the second side flow through rectifiers D1, D2, D3, and D4 on the secondary side in a current freewheeling mode, rather than through the circuits on the primary side.
However, the full-bridge DC-DC converters, constructed as shown in FIG. 6, have their respective problems. A saturation reactor SR included on the primary side experiences undesirable power loss and requires additional cooling devices. In addition, the insertion of an active element in the secondary side, as shown in FIG. 6(b), demands a control circuit, resulting in additional cost.
For the converter circuit of FIG. 6(c), the voltage V.sub.rec of rectifiers D1, D2, D3, and D4 on the secondary side is doubled by a resonance between the leakage inductance of the transformer T and the capacitor in an auxiliary circuit 10. This causes larger voltage stresses to diodes D1, D2, D3, and D4. There are other problems as well in requiring more elements.